Corvette Carburetor Restoration - Technically Speaking

Carburetor Tech: Time for a Rebuild?

James Berry Jul 1, 2011 0 Comment(s)
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One area that separates a specialized carburetor rebuilder from a do-it-yourselfer or local shop is the details. For example, when rebuilding the baseplate, Daytona Parts Company bores out each end of the plate where the throttle shaft runs and install a bronze bushing (Image B). The throttle shaft will also be nickel plated to return it to its original size (Image C). This is done due to the wear that occurs over years of use. When you accelerate, you're pulling the throttle shaft back toward you; over time this can cause the baseplate to become egg shaped and leak. New Teflon bushings are also installed, as necessary, on the throttle shaft during.

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When installing the throttle plates back into the baseplate, it's important to make sure that they move freely and that you can see a perfect “halo” around them (Image D). Once that's done, all of the throttle plate screws are installed with Loctite.

The primaries and secondaries will need to be synchronized by bending the connector link with a special tool (Image E).

Vehicle backfires often cause power-valve failures. These typically result when a vehicle is stored, and the bowls on the carburetor become dry. When the engine is cranked, it “pops” back through the carb. Daytona Parts Company helps reduce the chances of this happening by drilling a relief hole in your original baseplate (Image F) and installing an anti-backfire check valve (Image G).

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A professional rebuilder will remove all of the plugs from the secondary metering block so these areas can be cleaned of sediment (Image H).

Another concern with older Holley carburetors is that the float bowls may have become worn or stripped where the needle-and-seat adjustment is located. A professional rebuilder will bore out the float bowls where the float-adjustment screw is located and install a brass bushing so the customer can retain the original bowl (Image I).

Tension is removed from the secondary diaphragm spring by placing the diaphragm housing in a cup (Image J). This prevents the diaphragm, which is made from rubber, from becoming distorted upon assembly. Four different diaphragm-shaft lengths were used on Holley carburetors, so care is taken to ensure that the correct one is installed. If the diaphragm is too short, the secondaries they will not close correctly, and may even stay open all of the time. If the diaphragm is too long, the secondaries may never open at all.

After installing the secondary diaphragm assembly onto the main body, it's pressure tested to ensure that the secondaries open and close correctly (Image K).

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The accelerator pump will need to be adjusted to specification, to ensure that the proper amount of fuel is delivered under acceleration (Image L).

The choke assembly has several adjustments, including air gap and choke unload. It's important that your choke is working correctly for several reasons, including getting the rpm up to approximately 1,800 to achieve oil pressure as soon as possible. An incorrect rpm choke setting can cause an overly rich fuel mixture on cold startup, which can “wash down” the cylinder walls with fuel and foul the plugs.

We asked Ron Hewitt for his recommendations on keeping our freshly restored carburetor working efficiently for as long as possible. He offered the following suggestions:

- Avoid alcohol-laced fuel
- Keep as little fuel in the tank as necessary when the vehicle is in
- Use a fuel with the lowest octane level your car will allow
- Start your engine at least once a week and let it warm up to operating temperature.




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